Agate Fossil Beds National Monument, natural “depository” of an extinct animal community on the Niobrara River in northwestern Nebraska, U.S., 40 miles (64 km) north of Scottsbluff. The beds were laid down as sedimentary deposits about 20 million years ago (Miocene Epoch) and bear the remains of prehistoric mammals including Menoceras (two-horned rhinoceros), Moropus (7 feet [2 metres] at the shoulders with a horselike head), and Dinohyus (a large piglike beast). The site, named because of its proximity to rock formations containing agates, lies in the Carnegie and University hills. Established as a national monument in 1965, it covers an area of 5 square miles (13 square km). James H. Cook was the first white person to discover the fossil bones, about 1878, and he settled in the region in the 1880s.

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river rising near Lusk, in Niobrara county, eastern Wyoming, U.S., and flowing east across the High Plains, the northern edge of the Sand Hills, and the eastern plains of Nebraska to join the Missouri River near the village of Niobrara, Neb., at the South Dakota state line. The name is of Omaha and...

constituent state of the United States of America. It was admitted to the union as the 37th state on March 1, 1867. Nebraska is bounded by the state of South Dakota to the north, with the Missouri River making up about one-fourth of that boundary and the whole of Nebraska’s boundaries with...

earliest major worldwide division of the Neogene Period (23 million years to 2.6 million years ago) that extended from 23 million to 5.3 million years ago. It is often divided into the Early Miocene Epoch (23 million to 16 million years ago), the Middle Miocene Epoch (16 million to 11.6 million...